1. Field of the Invention
The present invention relates to an optical packet switching system that enables packet switching for each optical packet by switching an optical switch according to routing information given to an optical packet signal.
2. Description of the Related Art
In optical transmission systems employing wavelength division multiplexing (WDM), a technique that performs the path switching per wavelength by the use of a wavelength selective switch (WSS) and the like is put to practical use. As a technology that may succeed this technique, an optical packet switching method is now being investigated. In this optical packet switching method, an IP packet (10 GEther (10 Gigabit Ethernet (registered trademark) signal and the like), for example, is used as a small unit with which the switching is performed, and each is converted into the form of an optical packet and then the route is switched by an ultrahigh-speed optical switch (see Reference (1) in the following Related Art List, for instance).
The IP packet does not transfer any significant information in the absence of data therein, so that the bandwidth corresponding thereto is wasted. However, if the optical packet switching system is realized, then the time slot of a packet where data is absent can be occupied by another packet. Therefore, the optical packet switching system is considered a promising technology of the future which is capable of markedly enhancing the bandwidth usage efficiency of the transmission path.
3. Related Art List    (1) Japanese Unexamined Patent Application Publication No. 2008-235986.
FIG. 1 is a diagram for explaining values monitored by an optical power meter when optical packet signals are inputted to the optical power meter. The optical power meter generally detects a time-averaged power. Thus, if the signal is a continuous signal like a SDH (Synchronous Digital Hierarchy) signal, the values monitored by the optical power meter will be constant because control is performed such that mark rate is always 50%.
In contrast to this, with optical packet signals, the data amount and the packet length differ per packet therefore there are time slots where there are optical signals and time slots where there are not. Hence, the packet density varies in real time. The packet density is defined as a value indicating “packet length/packet interval” (the ratio of packet length over packet interval). FIG. 1 indicates a case where the packet density is high, a case where the packet density is low, and their intermediate case therebetween. The peak powers of optical packets in the respective cases are the same. However, since the values monitored by the optical power meter are those of time-averaged powers of light inputted to the optical power meter, they depend on their packet density. In other words, the monitored value is high if the packet density is high, but the monitored value is low if it is low.
In not only the optical packet switching system but also an optical transmission system, an optical level diagram is set to ensure optimal signal quality. For the optical transmission system that handles the continuous signal such as the SDH signal, the optical level diagram is preferably set using the time-averaged power. However, for the optical packet switching system handling the optical packet signals, it is necessary to set a packet density and a packet length that each serves as a reference, in order that the optical level diagram can be set using the time-averaged power. However, since it is necessary to define the varying range of constantly changing packet densities and packet lengths as a variation, it is not easy to set the optical level diagram using the time-averaged power. Thus, it is desirable in the optical packet switching system that the optical level diagram be set using a peak power independent of the packet density and the packet length.
Where the optical level diagram is set using the peak power, the peak power of the optical packet signal needs to be obtained to control the peak power of the optical packet signal in accordance with the optical level diagram set. As a method for detecting the peak power of the optical packet, there is available a conventional method using a WDM monitor. For example, the WDM monitor capable of responding on the sub-ns order is required to detect the peak power per 10 Gbps optical packet signal. Using such a WDM monitor in the optical packet switching system is not realistic or practical in view of cost and other factors.
Another method available for detecting the peak power is a method using an oscilloscope. FIG. 2 illustrates an optical packet signal detected by the oscilloscope. As shown in FIG. 2, an optical packet signal is comprised of a preamble and data. The preamble is of a pattern composed of alternating marks of 1's and 0's, and the peak power of the preamble is the true peak power of the optical packet signal. The power of a part of data where the marks 1's continue looks higher than the true peak power. The power of a part of data where the marks 0's continue looks lower than the true peak power. In this manner, although it is possible to detect the peak power when the oscilloscope is used, this method is not realistic or practical in the light of the fact that the oscilloscope must be used to measure during system operation and in the light of the oscilloscope being incorporated into the system.